OK pressed F8 went into safe mode OK and ran scannow. That reported some files repaired but other not. Restarted only to get stuck as before. Then when it booted correctly, at the third attempt, turned off the hibernate mode in powercfg. Since then two successful boot ups.
This problem started last week when I left the computer for a while, something I rarely do, and on return it has gone into sleep mode and I couldn't "awake" it. This XPS has a small LED screen in the top of the processor box through which, by adjacent control buttons, you run Windows Sideshow which "allows you view information from your computer regardless of whether it is on, off or sleeping" When the computer was "stuck" this would still work which leads me to think that rather than being locked up the computer had gone into a sleep mode.
Can't explain why it always seems to boot up on the third attempt though. Fingers crossed problem solved.